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Coastal navigation

The first requests for sea level information arose from the needs of inshore navigation and access to ports with substantial tidal ranges. There was a need for real-time or only slightly time-delayed measurements.

To meet this need, the information was obtained by direct vision:

Either from the tide staff available at the entrance to ports or along channels in the maritime portion of rivers;

Or, in the major seaports, daytime signals using cones and cylinders, and night-time signals using green, red and white lights.

More modern systems were developed in some estuaries where water depths measured at different tide stations were transmitted by radio to ships, in particular VHF voice radio (Port of Nantes - Saint-Nazaire, Port of Bordeaux , Port of Rouen).

The uncertainty in the water level has repercussions on the navigation margins, which are the safety margins between the seabed and the vessel. In addition, due to the costs of operating large ships (waiting on roadstead is very expensive) and the need to maximise returns on port facilities, the Marine Services and pilots are increasingly demanding when it comes to the quality of tide measurements. Currently the precision required is on the order of five centimetres (± 5 cm).